I've been using this combo (Amp2 mounted to a RPi 3A+), running with piCorePlayer, for about 15 months, with no problems whatever. The power supply (fed to the AMP2) is 20V/6A.
A couple of days ago I had to adjust some wiring, and so I had removed the boards from the box they live in, and handled them a bit. (The Pi lives in a ventilated box on the floor behind the speakers.)
The next time I used it, and ever since, playback has been at half speed. I tried reconfiguring pCP to use the headphone jack, and audio works fine in that case. When I configure back to "HiFiBerry DAC+ (and Pro, AMP2)", I'm back to half speed again. I've examined both boards (using a loupe) for damaged/missing components. I've swapped in a different RPi, with no change in behavior.
It's been a long time since I've been overly worried about static damage, and in this case (when I first moved/handled the boards) the power remained on and the boards were never separated.
Any ideas? Is my amp toast? :-/